1. Field of the Invention
The present invention relates to a thermal head and a method for fabricating the same and, more specifically, to a crystalline structure in a heating film of a thermal head.
2. Description of the Prior Art
There has been a thermal recording system which thermally records picture information by bringing into slidable contact with the recording medium a thermally recording head (thermal head) comprising heating resistor elements each selectively heated according to the picture information. As good-performance heating elements and heat-sensitive materials have recently been developed, increasing attention is now directed to this system.
The thermal head used in such a thermally recording system is divided into three types, that is, thick film, thin film and semiconductor types. Of these three types, the thick film and semiconductor type thermal heads are considered to need further researches at the current stage because heating resistors employed in these types are not completely known in many points including physical property change of the resistors with time when subjected to repetitive thermal shocks and high-temperature thermal pulses.
On the other hand, heating resistor employed in the thin film type thermal head has relatively short life when operated under severe conditions, but is suitable for high speed printing because of its small heat capacity. In addition, this type is good in the bonding property to its substrate and excellent in mechanical strength. For such reasons, the thin film type is getting attention as a demand for high speed printing increases.
The thin film type thermal head usually comprises an insulating substrate, a grazed layer formed on the substrate, a heating resistor layer provided on the grazed layer, a plurality of electrodes juxtaposed to each other on the resistive layer for supplying electric power to the layer, and a protective layer made of frictional-wear-resistant material coated on the electrodes.
The heating resistive layer is generally made of tantalum nitride (Ta.sub.2 N), nichrome (NiCr), tantalum-silicon dioxide (Ta-SiO.sub.2) or the like.
Of these materials, tantalum nitride, which does not exist in nature, is prepared by a reactive sputtering process. More specifically, tantalum as target is sputtered in an atmosphere of a mixture of argon and nitrogen gases to emit tantalum atoms. The atoms are reacted with the nitrogen gas to form a thin tantalum nitride film. The composition of the formed thin film varies depending on the partial pressure of nitrogen during the sputtering and the physical properties of the thin film vary correspondingly. Thus, characteristic values of the film depend largely on the structure of sputtering apparatus and film preparation conditions. For this reason, the thin film which is used as heating resistors has been prepared in a region in which specific resistance and resistance temperature coefficient (TCR) vary only to a small extent with respect to change in the nitrogen partial pressure, that is, in a flat region called a plateau as shown in FIG. 8 where the specific resistance and TCR are plotted with respect to the nitrogen partial pressure.
Meanwhile, when it is desired to drive the thermal head for high-speed printing, a large electric power though momentarily must be repetitively applied to the heating resistors of the thermal head, making of tantalum nitride as mentioned above rapidly deteriorated. For this reason, there has been strongly demanded a heating resistor that has a long operational life and a high operational reliability.